I upgraded from lion to mountain lion on my macbook air 2012.
instantly I noticed that the battery life was terrible.
Instead of 8 -9 hours I was getting 2-3 hours. under the same load.
I looked round the apple forums, and didnt find anything.
I found a post somewhere [sorry, cant remember where] that suggested I delete the desktop preferences plist file and restart the Dock.
it seems to have worked for me.
I now have 8 hours battery life again.
to apply this fix:
rm ~/Library/Preferences/com.apple.desktop.plist killall Dock
you will loose your wallpaper settings, but your battery life should be fixed.
please let me know in the comments if it works, and I might be able to get some evidence to send to apple for a fix!
I thought id put my reasoning in here incase you need convincing.
my guess is that its something to do with mission control or launchpad, though I did notice in the plist file itsself that there is a
Change = Never;
ChangeTime = 1800;
if the change = never is getting ignored, and the changetime is seconds, then every 30 mins each of your desktops refresh. probably not enough to have an impact on your battery, but if its ms, then every 1.8 seconds its refreshing.
that would be enough to kill your battery.
deleting the file resets it to have pretty much nothing in it. which seems to fit with my fix